1. Field of the Invention
The present invention relates to a motor driving device including a dynamic braking circuit.
2. Description of the Related Art
A motor driving device widely uses a dynamic brake which causes a short circuit across the input terminals of a motor to perform dynamic braking, at the time of anomalies such as an emergency stop or the occurrence of an alarm.
A dynamic braking circuit typically includes a dynamic braking circuit relay (to be simply referred to as a “relay” hereinafter) located across the input terminals of a motor, and a dynamic braking resistor connected to the relay. In dynamic braking, the supply of driving power to the motor is cut off, and thereupon the relay is closed to cause a short circuit across the input terminals of the motor (i.e., between the phases of the motor windings). Since the motor maintains a magnetic field flux even after electrical isolation from the power supply and, while rotating by inertia, serves as an electric generator, the thus generated current flows into the dynamic braking resistor through the closed relay and produces a deceleration torque in the motor. In this manner, the dynamic braking circuit can rapidly convert the rotational energy of the motor into Joule heat by dynamic braking resistance (and motor winding resistance) to perform dynamic braking.
In general, the relay in the dynamic braking circuit includes contacts, the opening and closing life of which considerably depends on the voltage applied across the contacts upon contact opening and closing. When the relay shifts from an open to closed state, a phenomenon called chattering (or bouncing) in which the surfaces of two contacts forming the relay repeat contact and separation in steps has taken place for a predetermined time, and then the contacts finally, stably come into contact with each other and settle in a closed state. However, attempting to close the interval between the contacts while a high voltage is applied across the contacts of the relay causes a spark between the contacts due to arc discharge during the chattering. Such a spark welds the contacts of the relay together or wears them, thus shortening the life of the relay. To replace the relay at an appropriate timing as well, it is important to accurately predict the life of the relay.
As described in, e.g., Japanese Unexamined Patent Publication No. H5-266290, one technique for predicting the life of the relay compares with each other the result of life prediction calculation based on the voltage or current applied to the relay and the count result of the number of relay operations and displays the life.
As described in, e.g., Japanese Unexamined Patent Publication No. H8-033195, another technique detects the phase of current flowing through the dynamic braking circuit during the dynamic braking operation and its magnitude in this phase, calculates the capacity of the dynamic braking circuit per dynamic braking operation period, and determines that the dynamic braking circuit is abnormal when the dynamic braking current is higher than the maximum current or the capacity of the dynamic braking circuit is higher than the maximum capacity.
As described above, relay life prediction according to the conventional techniques may preferably use a current detection unit which detects a current flowing through the relay and entails certain costs, thus making it difficult to achieve a compact device. In particular, the motor driving device may be preferably provided with not only a current detection unit for motor driving but also a separate current detection unit for relay life prediction in the dynamic braking circuit. This accordingly increases the cost and may involve a large motor driving device.
Further, since the life of the relay shortens every time an arc is generated due to chattering between the contacts of the relay in the above-described manner, the life predicted with no concern for damage inflicted by the arc is inaccurate.